It is possible that GST is not configured properly in your Shopify store.
If you sell products that include a GST component, your store settings should reflect this.
An example is provided below, please check out the Shopify documentation for more info.
Left menu bar, Settings (bottom option) -> Taxes (see screen capture)
After you have correctly set up the Taxes at the store level, you will also need to set each individual product. This is required in our platform even if you have defined an Australia tax of zero.
For a single variant product, the checkbox "Charge taxes on this product" is right below the Price in the Pricing section of the main product screen, as shown below:
For product with variants, you will need to go to the variant edit screen, the checkbox "Charge taxes on this product" is there below the price.
Please make sure that all variants are using the same setting, we do not differentiate the tax status at the variant level, thus it will promote the setting for one of the variants to the product level, in our system.